1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus, and more particularly, to a sheet feeding apparatus and an image forming apparatus in which sheets are separately fed by blowing air to the sheets.
2. Description of the Related Art
Conventionally, in image forming apparatus such as a copying machine and a printer in which an electrophotographic technology is employed, a toner image is formed on an image bearing member such as a photosensitive drum provided in an image forming portion, and the toner image is transferred onto a sheet. After that, the transferred toner image is fixed to the sheet by a fixing unit. Such an image forming apparatus includes a sheet storage portion which stores sheets and a sheet feeding apparatus which feeds one by one the sheets stored in the sheet storage portion. After being fed by the sheet feeding apparatus, the sheets are conveyed to the image forming portion.
By the way, in some conventional sheet feeding apparatus, air is blown to a sheet bundle stacked on a tray in the sheet storage portion so as to blow upper sheets of the sheet bundle upwards. In this way, the upper sheets are loosened one by one and sucked onto and conveyed by a suction conveyer belt. Note that, in a sheet feeding apparatus of such an air feeding type, in order to reliably suck the sheet by the suction conveyer belt, it is necessary to maintain a position of a topmost sheet, which is loosened by being blown upward by the air, to a predetermined level. Thus, a sheet surface detecting unit which detects the position of the topmost sheet is provided so that the position of the topmost sheet is maintained within a certain range through raising/lowering control on the tray and adjustment of airflow based on a detection result of the sheet surface detecting unit.
In this context, many of the sheet surface detecting units include a sheet surface detecting member to come into contact with an upper surface of the topmost sheet. By a detection sensor detecting a position of the sheet surface detecting member, the position of the topmost sheet is detected. Further, as described in U.S. Pat. No. 7,744,081, in some conventional sheet feeding apparatus, a plurality of suction conveyer belts are arranged in parallel with each other, and the sheet surface detecting member can be stored in an internal space between the suction conveyer belts. In this way, an image forming apparatus is downsized.
FIGS. 7A and 7B illustrate an example of the conventional sheet feeding apparatus of the air feeding type as described above. As illustrated in FIG. 7A, the conventional sheet feeding apparatus includes an air blowing portion 70 which blows air in a direction indicated by the arrow 69 so as to blow upward and loosen upper sheets of a sheet bundle PA stacked on a tray (not shown). Further, the sheet feeding apparatus includes a suction conveyance portion 50 which sucks and conveys the sheet thus blown upward, which suction conveyance portion 50 is arranged above the sheet bundle PA, and an intermediate conveyance portion 71 which conveys the sheet sucked and conveyed by the suction conveyance portion 50. The intermediate conveyance portion 71 includes intermediate guides 72 and 73 which guide the sheet and a pull-out roller pair 74 which conveys the sheet.
The suction conveyance portion 50 includes two endless suction conveyer belts 53a and 53b which are arranged parallel with each other and looped around pulleys 51a and 52a and travel in a direction indicated by the arrow 60. Further, the suction conveyance portion 50 includes a suction duct 54 which sucks a sheet P with a negative pressure through an opening portion thereof facing the sheets P, and which is arranged inside the suction conveyer belts 53a and 53b. Still further, the suction conveyance portion 50 includes a sheet surface detecting flag 58, which is supported by links 55 and 56 between the two suction conveyer belts 53a and 53b and is storable in a storage portion 57 provided in the suction duct 54 as illustrated in FIG. 7B. Yet further, a detection sensor S to be turned ON and OFF according to a turning position of the link 56 is arranged on an upstream side in a sheet conveying direction of the suction conveyer belts 53a and 53b. 
In the sheet feeding apparatus structured as described above, at the time of feeding a sheet, when the upper sheets of the sheet bundle are blown upward by airflow from the air blowing portion 70, a level of a sheet surface of a topmost sheet P1 of the upper sheets thus blown upward of the sheet bundle is detected based on a position of the sheet surface detecting flag 58. Based on the detection results, raising/lowering control is performed on the tray such that the level of the upper surface of the topmost sheet P1 falls within a predetermined sheet surface level range in which the topmost sheet P1 can be sucked onto the suction conveyer belts 53a and 53b. After the topmost sheet P1 has been positioned within the predetermined sheet surface level range, the inside of the suction duct 54 is brought into a negative-pressure state by a negative pressure providing unit (not shown). In this way, the topmost sheet P1 at a predetermined sheet surface level is sucked toward a sheet suction surface of the suction conveyer belts 53a and 53b. 
By the way, when the topmost sheet P1 is sucked toward the suction conveyer belts as described above, the sheet surface detecting flag 58 is stored into the storage portion 57 provided in the suction duct 54 as illustrated in FIG. 8 by a negative pressure provided by the negative pressure providing unit and by the topmost sheet P1 moving upward. In this way, the topmost sheet P1 is sucked onto the sheet suction surface of the suction conveyer belts 53a and 53b. Next, when the suction conveyer belts 53a and 53b is moved in the direction indicated by the arrow 60 in the state in which the topmost sheet P1 is sucked to the sheet suction surface, the topmost sheet P1 is conveyed integrally with the suction conveyer belts 53a and 53b. 
Then, the topmost sheet P1 conveyed by the suction conveyer belts 53a and 53b is guided along the intermediate guides 72 and 73. When a leading edge of the topmost sheet P1 reaches the pull-out roller pair 74 of the intermediate conveyance portion 71, the negative-pressure state in the suction duct 54, which is caused by the negative pressure providing unit, is cancelled. After that, rotation of the suction conveyer belts 53a and 53b is stopped, and the topmost sheet P1 is pulled out by the pull-out roller pair 74.
Then, the topmost sheet P1 thus pulled out of the suction conveyance portion 50 by the pull-out roller pair 74 is conveyed to the image forming portion. Subsequently, by repeating the same operation, the stacked sheets P are sequentially fed one by one to the image forming portion.
By the way, in the conventional sheet feeding apparatus of the air feeding type as described above, a blown-upward level of the sheet under the sheet surface control may be different between a part of the topmost sheet P1 against which the sheet surface detecting flag 58 is in abutment and a part of the topmost sheet P1 against which the sheet surface detecting flag 58 is out of abutment because the blown-upward condition is different. Specifically, at the part of the topmost sheet P1 against which the sheet surface detecting flag 58 is in abutment, the topmost sheet P1 is pressed from above, for example, by the weight of the sheet surface detecting flag 58, and hence the blown-upward level decreases. Such tendency is prominent especially when the sheet to be sucked and conveyed has low rigidity. For example, when the sheet is a thin sheet having low rigidity, as illustrated in FIG. 9A, a sheet surface level of the part of the topmost sheet P1 against which the sheet surface detecting flag 58 is out of abutment is higher than a sheet surface level of the part thereof against which the sheet surface detecting flag 58 is in abutment, and enters a blown-upward state in which the part of the sheet lifted by blown air is close to the suction conveyer belts 53a and 53b. 
When the topmost sheet P1 is sucked in such a blown-upward state, the part of the topmost sheet P1 against which the sheet surface detecting flag 58 is out of abutment and which is closer to the suction conveyer belts 53a and 53b reaches the suction surface earlier. As a result, as illustrated in FIG. 9B, the topmost sheet P1 is sucked onto the suction conveyer belts 53a and 53b in a state in which the part of the topmost sheet P1 against which the sheet surface detecting flag 58 is in abutment is deflected downward.
When the topmost sheet P1 is conveyed in such a state, as illustrated in FIG. 9C, a part T of the topmost sheet P1 which is deflected downward may collide against an inlet portion of the intermediate guide 73. As a result, the leading edge of the sheet may be damaged, or sheet jamming may occur. Further, the deflected part may be wrinkled at the time of passing a guide portion and a conveying roller pair on a downstream side in the sheet conveying direction.